Yeadonian Substage

Pot Clay
Yeadonian strata comprise only some 50 m in thickness, ranging from the Gastrioceras cancellatum Marine Band
at the base to the Gastrioceras subcrenatum or Pot Clay Marine Band above, which marks the start of the
Westphalian Stage (Coal Measures, Figs.1 and 2). Again, mudrocks form a significant part of the sequence,
and the Pot Clay near the top has been worked extensively, especially in the Loxley Valley, for making refractory
bricks for lining furnaces and, when mixed with another clay from Stourbridge, for crucibles for the Huntsman process
of making steel. Locally, a tough ganister is developed just above the Pot Clay (Fig. 3). An intermittent
thin coal seam, the Pot Clay Coal is also recorded.

Fig. 1: The Pot Clay horizon in an old fireclay working, Loftshaw, near Ughill

Rough Rock (up to 30 m thick)
The Rough Rock forms an extensive outcrop to the immediate west of Sheffield and continues to West Yorkshire, where
it is worked for a wide variety of building stone products. It is mostly a coarse-grained feldspathic sandstone,
with cross bedding, but it is locally more thinly bedded, sufficient to be known separately as the Rough Rock Flags.
Indeed roofing flags from the Rough Rock or Rough Rock Flags were highly prized in the days before the coming of Welsh
slate (Figs. 4 and 5). The unit is still actively worked near Huddersfield for paving stones, as seen in the
Peace Gardens in the centre of Sheffield.
The base of the Rough Rock is often erosional and in places it rests upon another marine band, Gastrioceras cumbriense.
The Rough Rock forms a dominant feature in the landscape and often marks the edge of the predominantly moorland or rural
land use before the built-up areas of Sheffield and Barnsley on the Lower Coal Measures to the east (Fig.
6).